Correcting show drain under slab?

Connecting shower drain under slab?

Hi all - this is my first post, but I've used this forum many times to get info for my DIY projects. Definitely the best DIY plumbing forum I have found!

I'm renovating a basement bathroom which previously had a toilet and a corner shower unit. We are enlarging the bathroom and moving the shower a couple feet further away from the toilet so I chipped out the floor to get to the drain plumbing.

I found the existing shower drain was a 1-1/2" pipe tied in directly to the toilet drain with no venting. I presume this is incorrect. There is also a what appears to be a drain tile pipe running beside the toilet drain pipe. Any idea why that would be there?

The following pic shows the existing plumbing and is marked up to indicate the desired location of the toilet flange. The new shower drain will be approximately 5' directly to the right of the toilet flange.

Obviously I will have to remove some more concrete to redirect the drain tile pipe to be able to relocate the toilet flange, but what is the correct way to connect my shower drain (I'll change the pipe to 2") to the existing plumbing and how should I vent it. Would it be correct to connect the drain as indicated in green? I have full access to the ceiling and all four walls.

Thanks

I think a little more concrete will need to be tapped out around the two stacks. That may help determine what the (third?) pipe is.
The shower needs to be vented before it wyes into a main branch. The way it was done before was wrong. Newer toilets require better waste and vents then before, simply because the flush is now so much quicker.

Waste pipes from above can't be used as vents for downstairs fixtures either.

Typically, a shower should be vented within five feet of the 2" p-trap. The vent can be 1-1/2"

Thanks for the response.

The mysterious third pipe under the slab must be some sort of drain tile because it has 1/2" holes in it. I just don't know why it's there. It does not tie into the waste water drain - it runs beside it. There is some sort of sump under the floor in another room in the other side of the basement. I presume it goes there, although I don't know what it is for.

I did some more digging around the two vertical pipes going into the slab. The 4" drain from the main floor elbows under the slab and goes to the left. The 2-1/2" pipe tees into the 4" toilet drain from the basement. Nothing drains into this pipe from above. It just runs beside the 4" drain pipe from the main floor and it connects into it about 4' above the main floor as shown in this diagram. I presume this is just a vent for the basement toilet drain.



Can I vent the shower by running a 1-1/2" pipe from between the shower p-trap and the toilet, running it under the slab to the wall, then up to the ceiling and over to this vent pipe as indicated above and below?

I had to chip out a bunch more concrete, but I made some progress (I hope). My house doesn't have any kind of backflow prevention, so I decided to add one while I've got this torn up. This was the only way I could think of to connect the toilet and show drain in the position I wanted it. Can someone please have a look at these pics and let me know if it looks ok (if not 100% correct).